I have to agree with Stefan. Why would a working fusion reactor quickly lead to fusion powered space craft when the working fission reactors we have had for the last 40 years have not yet lead to nuclear powered space craft? And scientists have been predicting we would have (Earth based) fusion power within 10 years for about 40 years now.

Actually they usually predict fusion power in 30 years! And yes terrestrial power plants would come first. In fact profits from them would fund the space applications.

This paper claims a pilot plant with a net power output of 100 MW in just 5 years. For $200 Million of engineering and development. The small amount of remaining research is only to determine the optimum configuration, and should cost a couple of million.

I've since discovered that EMC2, Bussard's company, still has a US Navy contract for the extra $2 Million, but Congress didn't fund that part of the Navy budget in 2006, (advanced propulsion). There is now a petition, and building pressure to release the remaining money.

Ok, if you read the paper and do a little follow up, you will discover that the Boron11-proton reaction advocated by Dr Bussard, not only produces less than 3% of the neutrons that the so-called "clean" D-He3 reation produces, but also alpha particles at 2 discrete energy levels, just above and below 3 MeV IIRC. This allows the option of capturing them in grids to produce direct electricity, at very high voltage, at up to 95% efficiency. No need for large, heavy shielding and heat conversion equipment.

Dr Bussard's goal all along was to make space travel more practical. These reactors would be light and relatively small, (a few metres in radius). He has designed propulsion systems that vary from high thrust at 1000 Isp to low thrust at 75,000 up to possibly 500,000 Isp. Even an air breathing version. Single stage to Mars would be possible.

I read Bussard's October article. I'm no physicist, but my impression is that he knows what he is talking about and they have made some real progress in solving the problems of IEC fusion. On the other hand, it is clear he was taking a strong advocacy position, so the prospects are not likely to be as rosy as he was portraying them. It sounds like something that should continue to be researched, but I'd like to hear some other experts weigh in before believing it's time to invest $200 million.

Besides the general boosterism, a couple of things were red flags to me. One is that he is talking about scaling up based on a seventh power law. That makes for a pretty big extrapolation, and intuition suggests some other, currently unforeseen, effects might come into play to ruin an extrapolation over such a large range. The second red flag is that they have only experimented with extremely short pulses, which means that there may be multiple problems with stable, continuous operation that have not yet been recognized.

After saying all that, I'm still excited by what they've apparently done so far, and it would be a shame if the feds don't find some other avenue to continue funding their research. It's a good hedge bet against the other approaches that we are currently pouring billions into.

"open issues" can also mean, "I'm not going to risk my reputation by saying anything definite"

The paper & Google talk give the impression that Bussard expects someone to cough up the $200 million, up front. Posts in other forums from associates say he is first looking for the $2 million to build the final test machines, WB7 & WB8, which should demonstrate much longer run times.

He then expects it to go before an independant review panel before anyone commits to the$200 million. Unfortunately, his enthusiastic nature makes him seem a little eccentric, but at age 80, we should allow him a litlle lattitude.

I shouldn't think he's trying to secure a cushy job for the next 10 or 20 years. (unlike the ITER crowd)

I read the document the initial post links to - and will read it again.

Regardless of the technology fusion requires energy from another source. This will hold also if the same technology - the one studied in that document in this case - would be used to propel space vehicles.

In so far the question needs to be considered what this other source will be or will have to be. At present the majority of electricity generated on Earh is got by non-fusion nuclear reactors - which are crfiticized by the environmentalists - and carbon and oil power plants - which are criticised also.

In space solar power seems to be required and the amount of hydrogen or other fuels for fusion is much less than on Earth.

Ekkehard, are you thinking of the energy needed to "prime the pump"? You would need a substantial source of electrical energy to charge up the grids and inject the initial population of electrons and ions in order to start up the fusion power plant.

A lot would depend on how much energy was needed. A spacecraft would almost have to rely on batteries or fuel cells, it seems. If the propulsion system is not going to be operating all the time you would also need some alternative way to power the electrical systems in the vehicle. Solar cells are only effective out to the orbit of Mars, so if you want to do missions to the outer planets they are of no help.

The obvious approach would be to draw off a little power while the propulsion system is operating to recharge the batteries or replenish the hydrogen and oxygen fuel cells by electrolysis. Whether that would work in practice depends on many things, such as how long the engines would run and how often, whether the power plant could operate in some "low power" mode, and how fast you could recharge the batteries. There's not nearly enough information here to answer any of those questions, I think. However, Bussard has evidently been publishing papers for years on ideas for designing fusion-powered spacecraft. Perhaps he has touched on some of these subjects.

The informations I have up to now seem to say that very much energy is required to achieve fusion. As far as I know scientists already succeeded in causing fusion but the amount of energy that has gone into it allways was that much higher than the enegry got that they say that economic application is way off.

Since the reactors applied in those experiments are large and very few fusions are achieved only it seems to me that a fusion reactor capable to propel a space vehicle would be really huge.

May be that the reactor talked about in the document WannabeSpaceCadet links to means a smaller reactor for propulsion but still a lot more fusion events will be required than achieved up to now in experiments.

And the talk necessaryly is about number of fusions per unit of time because most accelerations of vehicles need to be done over a larger amount of seconds. By number of fusions I am talking about number of atoms combined to one new atom.

It is of meaning how much kg of hydrogen must be consumed by fusion to increase the velocity of a vehicle by 1 km/s.

You are correct that all research fusion reactors to date have produced less energy than they required to operate. Of course, that's precisely what they have to fix before a fusion reactor can ever be used as a power source. They have to achieve "break even", and then far exceed it. Otherwise the whole contraption is nothing but a very expensive electric heater.

One characteristic common to both magnetic confinement and electrostatic confinement, evidently, is that the larger you make the machine the more favorable the energy balance is. That's why both Bussard and the ITER folks are talking about scaling up their reactors. Bussard claims that one advantage of IEC is that you can achieve net power generation with a much smaller machine than the tokamaks. He says he only needs to build a 2m diameter machine to produce 100MW of excess power. ITER (the next generation tokamak) will need to be ten times that big to break even. Cost seems to scale with size, too. Bussard wants $200 million to build his 100MW machine. ITER will cost $10 billion to produce 500MW.

Working fusion reactors, that generate electrical power, would obviously only need external power to start up. While large, that amount of power would only be needed for a very small amount of time, so a lower power source could charge up capacitors, as was actually done in Bussard's tests.

Fusion power could be used in many way to generate rocket thrust. Heating of hydrogen or another fluid (even air) would produce high thrust but lower Isp (about 1000). Electrical power for existing technology like ion, hall effect, arc jet or vasimr engines could produce Isp up to 10,000, but at lower thrust. Directed fusion product emission would produce Isp from 60,000 up to maybe 1,000,000, at low thrust.

At high thrust Isp 1000, 107 tonnes would produce 1 km/s for a 1,000 tonne vehicle. A SSTO would require 1.7 kg of fuel for each kg of vehicle & payload to reach orbit, less if it used air for the first part of the flight.